Materials which are used to form optical components, such as laser windows or optical fibers, must be transparent to the particular wavelength of radiation that they must transmit. The use, particularly of metal halides as windows for high powered lasers at 2-6 micrometers and 10.6 micrometers, requires rigid constraints on anion purity levels. Metal halide crystals grown from state-of-the-art purified starting materials contain trace cation and anion contaminants which, when subject to high energy laser applications, cause undesirable optical absorption and structural failures. Even materials having purities of 99.999% form windows which have an undesirable tendency to absorb energy from the laser beam. This absorption of energy can cause the window to overheat, resulting in fracture and opacity.
Anion purity, therefore, is a primary concern for high-power IR window materials since anions, particularly OH.sup.- and O.sup.--, contribute significantly to IR absorption. The vibrational modes of anions are infrared active and often involve high absorption cross-sections so that much less than one ppm is needed to achieve an absorption coefficient below 0.001 cm.sup.-1 in the crystal.
Metal fluorides such as thorium fluoride (ThF.sub.4) have recently been found to be useful for, among other things, thin film reflectors and anti-reflectors which are suitable for use in high-power carbon dioxide laser systems. When used as a reflector, thorium fluoride is provided as a thin film on a suitable substrate that is external to the laser resonator cavity. This film can thereby deflect the laser beam in a predetermined direction toward the target. It is desirable to deflect the laser beam efficiently, in order to prevent losses in the laser beam intensity. When used as an anti-reflector, materials such as thorium fluoride may be coated on the surface of a laser window to provide a refractive index at the window surface such that the reflection of the laser beam is minimized while the transmission of the laser beam through the window is maximized. However, in order to be suitable for such purposes, thorium fluoride must have a high transmission and low absorption for the 10.6 micrometer radiation from the carbon dioxide laser so that the film will not heat up enough to cause its own destruction, as discussed previously.
Commercial powders currently available are unsuitable starting materials for the congruent growth of certain metal fluorides, particularly those such as ThF.sub.4. The anion purity of these powders may be no better than three-nines complete in the conversion to the fluoride. A few hundred ppm of oxide or hydroxide in rare-earth or alkaline-earth fluoride powders cause difficulties in crystal growth. However, even if the anion purity is satisfactory after conversion, an alternate problem such as the stability of the powder is encountered. In particular, the powder can undergo hydrolysis as a result of the absorbance of moisture from the air.
Several methods are given for the conversion of metal oxides to metal halides. One method which involves treatment with anhydrous HF, a method capable of achieving four nines conversion, encounters two difficulties. First, the large amount of water formed renders HF vapor very corrosive, and therefore there is a tendency for the metal halide to pick up further impurities. The exothermic reaction has a runaway tendency which thwarts further conversion by confining the reaction to the surface, resulting in the formation of a crust.
Another more effective procedure, which combines the wet and dry conversions, is disclosed by R. C. Pastor and R. K. Chew, entitled "Process for the Preparation of Ultrapure Thorium Fluoride", U.S. Pat. No. 4,519,986 filed on Jan. 28, 1982, which is assigned to the present assignee. Thorium oxide is reacted with a predetermined amount of hydrofluoric acid to form a solid reaction product which is then dried under controlled heating to form hydrated thorium fluoride. The hydrated thorium fluoride is then exposed to a reactive atmosphere comprising hydrofluoric acid vapor and a chosen fluoride compound in the gas phase, utilizing an inert helium carrier gas at elevated temperatures. The hydrated thorium fluoride is exposed to this reactive atmosphere for a selected period of time to remove substantially all of the water and water-derived impurities from the hydrated thorium fluoride. This process is particularly useful in the production of heavy metal fluorides in the crystal form.
Metal halides have been purified by numerous other prior art methods. For example, U.S. Pat. No. 3,826,817, assigned to the present assignee, discloses a method for the synthesis of metal halides having extremely low hydroxyl ion contamination levels. These metal halides are synthesized by reacting an alkali salt in the solid state with a gaseous compound that is capable of simultaneously replacing the anion of the salt with a halide and gettering any water that might be produced by the chemical reaction.
U.S. Pat. No. 3,969,491, assigned to the present assignee, discloses a process wherein alkali metal halides in the molten form are scrubbed with gaseous nascent halogen, preferably a halogen corresponding to the halide anion. Once the gaseous nascent halogen has removed the trace impurities of both cations and anions, the purified material can then be utilized to form single crystals.
U.S. Pat. No. 3,932,597, assigned to the present assignee, discloses a process wherein metal halides are scrubbed with a halide-source species in the vapor phase to upgrade their purity. This process is effective in not only reducing the concentration of oxyanion impurities and volatile cation halide impurities, but it is also effective in reducing hydroxyl ion contamination as well.
U.S. Pat. No. 4,190,640 is an improved process for generating nascent bromines through the pyrolytic dissociation of CBr.sub.4. This patent provides a reactive gas carrier comprised of a mixture of an inert gas such as nitrogen, argon or helium and nitric or nitrous oxide in the bromide pyrolysis chamber as the bromide is subjected to temperatures in excess of 600.degree. C.
Though numerous attempts have been made to achieve ultrapure metal fluorides, the demands of the art mandate an ever increasing need for the purest material possible.